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Title: Electronic excitation of CO by positron impact

Abstract

We report calculated cross sections for the electronic excitation of carbon monoxide by positron impact. The calculations were carried out with the Schwinger multichannel method and included six collision channels, namely the ground (X {sup 1}{sigma}{sup +}) and five electronically excited (A {sup 1}{pi}, I {sup 1}{sigma}{sup -} and D {sup 1}{delta}) states. Present calculated excitation cross sections to the A {sup 1}{pi} state did not present any resonant features, being in this sense consistent with previous calculations for the isoelectronic nitrogen molecule. The experimental a {sup 1}{pi}{sub g} excitation cross section of N{sub 2} presented a resonantlike structure which would also be expected in the A {sup 1}{pi} excitation of CO. We discuss possible reasons for the disagreement between experiment and theory.

Authors:
; ;  [1];  [2]
  1. Departamento de Fisica, Universidade Federal do Ceara, 60455-760 Fortaleza, Ceara (Brazil)
  2. (UNICAMP), 13083-970 Campinas, Sao Paulo (Brazil)
Publication Date:
OSTI Identifier:
20786314
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. A; Journal Volume: 72; Journal Issue: 6; Other Information: DOI: 10.1103/PhysRevA.72.062715; (c) 2005 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
74 ATOMIC AND MOLECULAR PHYSICS; CARBON MONOXIDE; CROSS SECTIONS; EXCITATION; EXCITED STATES; GROUND STATES; MOLECULES; NITROGEN; POSITRON-MOLECULE COLLISIONS; POSITRONS

Citation Formats

Silva, Euclimar P. da, Varella, Marcio T. do N, Lima, Marco A. P., and Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas. Electronic excitation of CO by positron impact. United States: N. p., 2005. Web. doi:10.1103/PHYSREVA.72.0.
Silva, Euclimar P. da, Varella, Marcio T. do N, Lima, Marco A. P., & Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas. Electronic excitation of CO by positron impact. United States. doi:10.1103/PHYSREVA.72.0.
Silva, Euclimar P. da, Varella, Marcio T. do N, Lima, Marco A. P., and Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas. Thu . "Electronic excitation of CO by positron impact". United States. doi:10.1103/PHYSREVA.72.0.
@article{osti_20786314,
title = {Electronic excitation of CO by positron impact},
author = {Silva, Euclimar P. da and Varella, Marcio T. do N and Lima, Marco A. P. and Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas},
abstractNote = {We report calculated cross sections for the electronic excitation of carbon monoxide by positron impact. The calculations were carried out with the Schwinger multichannel method and included six collision channels, namely the ground (X {sup 1}{sigma}{sup +}) and five electronically excited (A {sup 1}{pi}, I {sup 1}{sigma}{sup -} and D {sup 1}{delta}) states. Present calculated excitation cross sections to the A {sup 1}{pi} state did not present any resonant features, being in this sense consistent with previous calculations for the isoelectronic nitrogen molecule. The experimental a {sup 1}{pi}{sub g} excitation cross section of N{sub 2} presented a resonantlike structure which would also be expected in the A {sup 1}{pi} excitation of CO. We discuss possible reasons for the disagreement between experiment and theory.},
doi = {10.1103/PHYSREVA.72.0},
journal = {Physical Review. A},
number = 6,
volume = 72,
place = {United States},
year = {Thu Dec 15 00:00:00 EST 2005},
month = {Thu Dec 15 00:00:00 EST 2005}
}
  • We present cross sections of electronic excitation of nitrogen molecule (N{sub 2}) by positron impact using the Schwinger multichannel method. Our calculated cross sections are compared with the recent and the only available experimental data of Sullivan et al. [Phys. Rev. Lett. 87, 073201 (2001)]. Present theoretical results for excitation to the a {sup 1}{pi}{sub g} states do not reproduce the near-threshold structure observed in the experimental data.
  • Absolute measurements are presented for the positron-impact cross sections for positronium formation, direct ionization, and total ionization of the diatomic molecules N{sub 2}, CO, and O{sub 2}, in the range of energies from threshold to 90 eV. Cross sections for the electronic excitation of the a {sup 1}{pi} and a{sup '} {sup 1}{sigma} state in N{sub 2} and the A {sup 1}{pi} state in CO near threshold are also presented. The experiment uses a cold, trap-based positron beam and the technique of studying positron scattering in a strong magnetic field. In O{sub 2}, a feature previously seen in the totalmore » ionization cross section is observed in both the positronium formation and total ionization cross sections. The possible origin of this feature and its relationship to positron-induced dissociation is discussed. In N{sub 2}, the near-threshold electronic excitation cross section is larger than that for positronium formation. This likely explains the relatively high efficiency of this molecule when used for buffer-gas positron trapping.« less
  • Recent experiments on electronic excitation of molecules by positron impact have shown much larger cross sections than in the electron scattering case. The challenge of understanding the origin of this enhancement, especially just above electronic excitation thresholds, motivates the search of theoretical explanations of the phenomenon as well as new experimental efforts to confirm the data. In an earlier theoretical effort, an application of the Schwinger multichannel method at two-state level of approximation for the X {sup 1}{sigma}{sub g}{sup +}{yields}B {sup 1}{sigma}{sub u}{sup +} electronic excitation of the H{sub 2} molecule, gave cross sections with smaller magnitude but reasonable qualitativemore » agreement with the experimental data. The purpose of this work was to study the numerical stability of the previous calculation and to investigate the influence of open channels (energetically accessible electronic states--multichannel effects) and closed channels (energetically inaccessible electronic states--polarization effects) on this transition. Our results show minor multichannel effects and a significative enhancement of the X {sup 1}{sigma}{sub g}{sup +}{yields}B {sup 1}{sigma}{sub u}{sup +} cross section at the threshold due to polarization effects if compared to the usual static results.« less
  • We have measured the first state-resolved, absolute cross sections for positron excitation of electronic states of an atom or molecule using a high resolution ({Delta}E{approx}25 meV FWHM) beam of positrons from a Penning-Malmberg trap. We present cross sections for the excitation of the low-lying levels of Ar, H{sub 2}, and N{sub 2} for incident positron energies between threshold and 30eV. For Ar and H{sub 2}, comparison can be made with theoretical calculations, and, in the case of H{sub 2}, the results resolve a significant discrepancy between the only two available calculations.
  • Electronic excitation of N/sub 2/ and CO condensed (i.e., physisorbed) on a metallic substrate has been investigated by high-resolution electron-energy-loss (HREEL) and low-energy electron-transmission (LEET) spectroscopies in the incident energy range 0--19 eV. The HREEL results yield a more complete picture of the spin-forbidden vibronic bands of the solids than previously available from photon spectroscopies. All levels are found to be shifted down by a few tens of meV and broadened with respect to the gas-phase values. As expected, Rydberg states are absent from such spectra. The LEET experiments provide a measurement of inelastic transitions near the electronic excitation threshold.more » By recording the second energy derivative of LEET spectra, transitions having a strong cross section near threshold are identified by the sharp structure they produce. These are the a /sup 3/Pi state in CO and, in N/sub 2/, the A /sup 3/..sigma../sub u//sup +/ and B /sup 3/Pi/sub g/ states and a new previously unobserved vibronic band near 12 eV. The magnitudes of thresholds cross sections in the solid phase are discussed in terms of single-electron--molecule scattering mechanisms.« less